Hot-electron transistor designs have evolved over the past two decades. An example of an early design of a hot-electron transistor is dislcosed in U.S. Pat. No. 4,616,241, entitled, "High-Speed Semiconductor Device", and issued to Naoki Yokoyama. The structural elements which generally define these hot electron transistors are an emitter layer, a base layer, a collector layer, an emitter potential barrier layer disposed between the emitter layer and the base layer, and a collector potential barrier disposed between the base layer and the collector layer.
In operation, a voltage bias is applied between the emitter and the base. The function of the emitter potential barrier is to limit the amount of emitter current at a given emitter voltage. By adjusting either the thickness or the height of the emitter barrier, the desirable emitter current level can be obtained. A low emitter barrier allows high energy electrons to penetrate the base while filtering out lower energy electrons, thereby limiting the current penetrating into the base. On the other hand, a high barrier inhibits tunneling of electrons. Once the high energy or "hot electrons" (herein defined as thermally non-equilibrium electrons) penetrate the emitter barrier, these electrons gain energy from the applied electric field and travel through the base to reach the collector potential barrier.
Heretofore, it has been generally assumed that the transmission coefficient of the electrons through the barrier is zero when the hot-electron energy is less than the collector potential barrier height and equal to one if the hot-electron energy is higher than the collector barrier height. This assumption concerning the basic operation of the device, however, is based on the theory that the hot-electrons experience both elastic and inelastic scatterings in the base and therefore, the propagation of hot-electrons does not exhibit quantum-mechanical characteristics. If this is the case, the phases of the hot-electrons will become incoherent and the subsequent motion of the hot-electron will be characterized by uncorrelated phases. Accordingly, prior art designs of collector potential barriers ignored the wave nature of the electrons. As a consequence, the collector potential barrier in previous designs have conventionally been made of a single material layer which serves as an electron energy filter. (See Yokoyama, U.S. Pat. No. 4,616,241).
The conduction-energy-band diagram for a prior art hot-electron transistor device is illustrated in FIG. 1 wherein the emitter to base voltage is labeled V.sub.eb. This figure illustrates the relationship among the essential structural elements of a hot-electron transistor which include: emitter 1, base 3, collector 5, and two potential barriers, an emitter potential barrier 2 (EPB) and a collector potential barrier 4 (CPB) 4. It should be noted that in this diagram the thermal electrons are assumed to follow a Boltzman distribution whereas the high-energy hot-electrons follow the distribution curve shown in FIG. 1. An important feature of prior art hot-electron transistors, as noted above, is that the collector potential barrier is composed of a one material layer. Thus, only the electrons with energy higher than the barrier height of the filter could overcome the barrier and be collected in the collector. The electrons with lower energy, however, would be blocked by the filter and drained through the base.
FIG. 2 is a graph of the current transfer ratio versus the emitter to base voltage of the hot-electron transistor previously described. It should be noted that a similar graph results from all prior art designs. The current transfer ratio is defined as I.sub.c /I.sub.E, where I.sub.c is the electron current flowing out of the collector and I.sub.E is the electron current flowing into the emitter. The current transfer ratio is measured under the condition that the emitter bias V.sub.eb is increased while the collector to base bias V.sub.cb is kept constant. As the emitter bias increases, the energy of the hot-electrons injected into the base increases. The portion of the hot-electrons that can overcome the collector potential barrier after traversing through the base will also increase. As a result, the current transfer ratio increases steadily with an increase in emitter bias for a large range of emitter bias.
Those skilled in the art know, however, that there are small oscillatory features superimposed on this smoothly rising background. These oscillations are demonstrated by a graph of the quantum-mechanical transmission coefficient of the collector potential barrier, as shown in FIG. 3. Here, FIG. 3 shows the oscillatory features of the transmission coefficient above the potential barrier height, which is 165 meV in this example. This may be compared to the transmission coefficient of unity above the collector potential barrier height and zero below it. Thus, it is the quantum mechanical oscillations of the transmission coefficient which explain the oscillatory structures on the graph of the current transfer ratio. These oscillations, however, eliminate the possibility of providing for a constant increase in the current transfer ratio over a range of voltages. Despite this disadvantage, these oscillations were ignored in prior art hot-electron transistor designs. The present invention addresses this problem.